1. Field of the Invention
This invention relates generally to the field of high-frequency electrical circuit, and particularly to a variable resonator which permits the resonant frequency to be set at any desired frequency and a variable phase shifter capable of varying phases of signals arbitrarily.
2. Prior Art
In the art of wireless communication utilizing high frequency signals, discrimination of desired signals from unnecessary ones is made by taking signals in a particular frequency band out of a great many signals at all frequencies. The circuit performing this function is called filter and is mounted on many wireless communication apparatuses. Such filters are invariable in the center frequency and bandwidth which are major design parameters. Where a plurality of frequency bands and various frequency bandwidths are used in the wireless communication apparatus utilizing such filters, it is conceivable to provide a plurality of filters for the frequency bands and bandwidths to be used and switch from one filter to another by means of switches. In this instance, however, the problem is that the apparatus is upsized due to the size of the circuitry being enlarged. In view of this problem, the prior art has heretofore conceived various approaches to making variable the resonant frequency of the resonator which is one of the components of the filter in order to realize filters which provide means for varying the center frequency and bandwidth.
Japanese Patent Application Publication No. 6-61092 (literature 1), for example, discloses constructing a resonator from a condenser composed of parallel planar plates and an inductor such that the resonant frequency may be varied by mechanically changing the spacings between the parallel planar plates. This is an example in which a variable resonator is constructed by using lumped-constant circuit elements.
Further, the resonator utilizing a microstrip transmission line which is a distributed-constant circuit is also known as per “The Lecture of Practicable Microwave Technology,” Vol. 3, pp. 24-25, pp. 48-49, pp. 199-200 and pp. 219-221 (literature 2). Specifically, FIG. 1 illustrates the construction of the end short-circuited λ/4 resonator according to the prior art; FIG. 1A being a plan view, FIG. 1B a cross-sectional view taken along the line A1-A1′ in FIG. 1A and FIG. 1C a side view seen from the line A2-A2′ in FIG. 1A.
This resonator 210, as shown in FIG. 1A, comprises a microstrip line 213 formed of silver on a dielectric substrate 212 having a ground conductor 211 on its back side. The microstrip line 213, having a constant line width W and a length L, has its one end 213a short-circuited with the ground conductor 211 at an edge of the dielectric substrate (see FIG. 1C) and the other end 213b connected with a transmission line 114.
FIG. 2A illustrates the electric current distribution in the microstrip line 213 of this prior art example. As will be seen from the drawing, the current is concentrated most on the edge portion of the microstrip line 213.
FIG. 2B is a graph showing the simulation results of the reflection coefficient in this prior art example, wherein the frequency having the minimum reflection coefficient is the resonant frequency.
With regard to the phase shifter, in an antenna device having a plurality of antennas and adapted to enhance the directivity by inputting signals with varied phases to the respective antennas, it is required to control the phase of signals to be input to the respective antennas in order to vary the directivity arbitrarily, and hence there is a need for a variable phase shifter capable of changing the phase arbitrarily.
Japanese Patent Application Publication No. 6-216602 (literature 3), for example, proposes that a microstrip line formed on a ferroelectric substrate be used to vary the phase difference between input and output signals by applying a voltage to the ferroelectric substrate to change the dielectric constant to thereby vary the wavelength of the signal passing through the microstrip line.